Fast and high-voltage electric pulses are needed within many technical fields, substantially for either simulating current and voltage pulses which may damage electric equipment, or for feeding equipment with pulses suitable for its function, for example piezoelectric tranducers which convert a voltage pulse into an acoustic pulse.
Fast electric pulses, that is, pulses with a large time derivative of the voltage dU/dt, and hence in most cases also a large current derivative dI/dt, give rise to interference effects, which both influence the shape of the pulse itself and other equipment through one or more of the following effects:
capacitive currents caused by the voltage derivative PA1 induced voltage caused by the current derivative PA1 electromagnetic radiation caused by the current and/or voltage derivative PA1 reflections of the impulse at locations with a changed characteristic impedance.
Some of the effects on the pulse shape are reduced steepness (increased rise time), ringings (superimposed oscillations), and reflected signals which may be both superimposed on the pulse itself and arrive after the actual pulse is already terminated.
Interference on the equipment may, for example, entail distortion of a measured signal, disturbance of its function, or even permanent damage such as electric breakdown of the insulation, or destruction of a semiconductor element by overvoltages.
The above-mentioned interference effects may be reduced by a combination of suitable electromagnetic shielding, overvoltage protective means, and careful design of the pulse generator. The knowledge and the technique within this field are well-established. In one embodiment of such an impulse generator, the coaxial cable is grounded in the cable screen whereas one end of the inner conductor is connected to a direct-voltage source with a high-ohmic charging resistor. The other end of the cable is connected to a load via a spark gap acting as a quick-closing electric contact. All the way from the end of the cable through the spark gap to the load, the signal path is designed with such a geometry that the same characteristic impedance as that of the cable is retained. If the load itself then terminates with the same resistance, the pulse is not reflected back into the cable. Also in case of an otherwise careful design, such a generator provides high-quality square pulses.
However, the above-mentioned embodiment has the disadvantage that it needs to be terminated with a load which has the same resistance as the characteristic impedance (matching) of the coaxial cable. A typical and very common value is 50 ohm. The actual load is often a high-impedance load, for example an insulation material, and must therefore be connected in parallel with a terminal resistance of the same value as the characteristic impedance. Then the voltage across the load is halved, so that the pulse generator must be dimensioned for twice the voltage across the load, which is a great disadvantage both from the point of view of cost and space.
For a non-matching load, a matching termination at the other end of the cable is needed instead. In this case it is suitable, instead of charging the inner conductor to high voltage, to connect it via a matching resistance to ground and instead charge the screen of the coaxial cable via a high charging resistance. If the screen is then connected to ground, a square pulse is formed which, exactly as in the above-described generator, is determined in its duration by the velocity of wave propagation and the length of the cable. The load need no longer be matched. The reflected pulse then arising is fully consumed in the matching resistance at the other end of the cable and thus cannot lead to an additional voltage pulse across the load.
A disadvantage with this type of pulse generator is that it is more difficult to minimize disturbing influence and pulse distortions than for the first-mentioned type. By energizing the screen itself, it has lost the shielding properties it possessed in the design with an energized neutral conductor. The noise level can be minimized by placing the cable in a shielding container, but it remains at an undesired high level which affects the accuracy and the quality of use.